Continuous selection in modern genetic programmes has led to sows that get more predictable in performance. The traits mentioned are traits with a large variation, which is only partly heritable. A trait like litter size has a heritability of 10%, varying from 6-12% in most research. So about 90% of the variation in litter size is due to other factors that do not follow normal additive genetic inheritance patterns.
Although the focus on this trait is in maternal lines and the trait is only expressed in females, there is another genetic factor that might also influence fertility. In most research studies the service sire effect was found to be very small compared to the maternally expressed part of litter size connected to ovulation rate. Although much smaller, only having a heritability varying from 1-3%, it might be desirable to be aware of its existence and the impact it can have on a farms production performances.
Boars might express lower fertility due to fever, some diseases or vaccinations can cause serious problems with temporarily or permanently reduced fertility. Most of the time libido related or semen quality parameters are also affected.
Use of mixed semen can partly or completely cover the impact of the presence of a boar being a carrier of reciprocal translocations impacting fertility.